Quantum-Chemical Modeling of Free-Radical Polymerization Michelle L. Coote* Introduction Free radical polymerization is a complex multi-step process. Even in its simplest form it comprises discreet propagation, initiation and bimolecular termination reactions, and in most practical systems a variety of additional reactions, such as various types of chain transfer processes, also occur. When one also considers that the rates of these individual reactions are often chain length dependent to varying extents, and that in industrially important processes more than one type of monomer and/or additional reagents are often present, it becomes clear that obtaining accurate and precise measurements of the rate coefficients of the various individual reactions can be a challenge. In recent years, the development of elegant laser-flash photolysis-based tech- niques, such as pulsed laser polymerization (PLP) and time- resolved PLP, has enabled the primary rate coefficients in simple homopolymerization systems to be measured with relatively few model-based assumptions. [1] Nonetheless, the model-free determination of the individual rate coefficients in more complex systems, such as copolymer- ization or controlled radical polymerization, remains elusive. Moreover, even in homopolymerization systems, the application of these techniques can be hampered if, for example, the monomer absorbs significantly at the wavelength of the laser. Computational quantum chemistry has the potential to solve these problems as it enables the rate coefficients of isolated individual reactions to be calculated directly, without recourse to kinetic model-based assumptions. It also provides access to detailed mechanistic information, such as transition state geometries, charge distributions and spin densities, which can be useful in interpreting the results and hence applying them in the rational design of new reagents. However, radical reactions generally require high levels of theory for accurate results and, since their cost scales exponentially with the size of the system, such accurate procedures are impractical for polymeric molec- ules. Instead it is necessary to use small model reactions to study the kinetic behaviour of polymeric molecules, and, so that such models can be as realistic as possible, choose relatively low-cost theoretical procedures for their study. Both the use of small model reactions, and the choice of theoretical procedures can be a potentially large source of error. Nonetheless, aided by rapid and continuing increases in computer power, computational chemistry is rapidly establishing itself as an accurate and useful kinetic tool Feature Article M. L. Coote ARC Centre of Excellence for Free-Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia E-mail: mcoote@rsc.anu.edu.au This article reviews recent progress in the application of quantum chemistry to radical polymerization processes, with a principle focus on establishing the current ‘best-practice’ methodology for obtaining chemically accurate calculations. The scope and limitations of computational chemistry for this field are also dis- cussed, and some of its leading applications in the areas of ab initio kinetic modeling and computer-aided reagent design are highlighted. 388 Macromol. Theory Simul. 2009, 18, 388–400 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/mats.200900050